protecting/unprotecting cell(s)

[Mel Seder <melseder yahoo com>]

a)I build a sheet.
b)I test it.
c)I want to protect all cells except the one(s) that requires input.

How do I go about c)?  Im sure steps must be taken in a certain order. 
Can you please tell me how?
 

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The true measure of a man is how he treats someone who can do him 
absolutely no good. -Samuel Johnson, lexicographer (1709-1784)